The present invention relates to a micromechanical component comprising a movable portion which can be caused to carry out resonant oscillations, i.e. an oscillating body. In particular, the present invention relates to micromechanical components of such a nature that an oscillation of the oscillating body out of the chip plane, i.e. vertically to the chip plane, is produced.
The functionality of certain micromechanical actuators, e.g. actuators designed to deflect of light, is based on the deflection of a movable element out of the chip plane. The movable element is connected to the rest of the chip via a bending or torsion spring. In order to achieve deflection, it is, on the one hand, known to implement the movable part as an electrode and to use a suitable counterelectrode so that the actuator will be driven by an electric torque. On the other hand, it is known to provide the movable part in the chip plane with conducting tracks in the form of a coil so that, when current flows through the conducting tracks in the magnetic field, a magnetic torque can be used for the purpose of driving.
In many micromechanical components the counterelectrode is arranged below the movable electrode so that, upon application of a voltage, an electric torque can be used for deflection from the chip plane. For producing such components, two fundamental approaches to a solution exist.
According to the first approach, the substrate, which can serve as a counterelectrode itself, or a counterelectrode structured from metal and arranged on the substrate has first applied thereto a sacrificial layer consisting e.g. of silicon dioxide or of a photoresist. On this sacrificial layer the movable electrode is then formed making use of a suitable conductive material, e.g. aluminium or a heavily doped polysilicon. Finally, the sacrificial layer is removed. M. Fischer describes e.g. in xe2x80x9cElectrostatically deflectable polysilicon torsional mirrorsxe2x80x9d in Sensor and Actuators A 44 (1994), pp. 83 et seq., a method in which the silicon substrate defines the counterelectrode, the sacrificial layer consists of silicon and the movable electrode is produced from heavily doped polysilicon. In the case of this method the electrode gap is determined by the thickness of the sacrificial layer so that electrode gaps having an arbitrary size cannot be realized by means of this method or similar methods. Hence, actuators having large lateral dimensions and a large deflection angle cannot be produced.
The second fundamental approach to a solution is based on a hybrid set-up. According to this approach, the counterelectrode and the movable part are produced separately from one another and combined towards the end of the production process. For example, H. Lxc3x6we describes e.g. in xe2x80x9cMexcex2technische Anforderungen bei der Herstellung von Silizium-Mikrospiegelnxe2x80x9d in Sensor 95 (congress volume), pp. 631 et seq., a set-up comprising a silicon frame in which the movable part is supported and a supporting plate which includes the counterelectrode. By means of this approach to a solution large electrode gaps can be realized. This approach to a solution entails, however, the disadvantage that suitably high driving voltages are required. In addition, the separate production of the micromechanical structure and of the supporting plate make the process complicated and expensive.
In the above-mentioned components, in which the counterelectrode is arranged below the movable electrode, the amplitude of the oscillation of the movable part can, of course, not exceed the size of the electrode gap. When a larger electrode gap is used for increasing the amplitude, it will, on the other hand, also be necessary to increase the driving voltage so as to be able to couple in the necessary energy per oscillation cycle. It is therefore not possible to achieve by means of this set-up a large amplitude making use of low driving voltages.
S. Miller describes e.g. in xe2x80x9cScaling Torsional Cantilevers for Scanning Probe Microscope Arrays: Theory and Experimentxe2x80x9d; in Transducers ""97 (1997) pp. 445 et seq., a set-up in the case of which the movable part of the micromechanical component can be deflected out of the chip plane without making use of a counterelectrode which is arranged below the movable electrode. The set-up described in this publication is provided with a pivotably supported cantilever which can be driven by capacitive actuators. The cantilever is provided with a plurality of interdigitated electrodes for this purpose. These movable interdigitated electrodes define together with the fixed interdigitated electrodes the capacitive actuators which take advantage of the so-called electrostatic comb drive levitation effect. In the case of this effect, a vertically asymmetric field acts on the movable electrodes and this results in a vertical force which causes the movable electrodes and, consequently, the movable cantilever to move away from the substrate. This effect can be utilized for causing static displacements as well as oscillations of the cantilever. In order to achieve the asymmetric field distribution, which causes the cantilever to move out of the chip plane, it will always be necessary that a substrate is provided adjacent the arrangement described, since, neglecting manufacturing tolerances, a torque permitting a deflection out of the chip plane would otherwise not exist. Only the existence of the substrate changes, when there is a suitable potential distribution, the field pattern such that a vertical force component will be created, which can be used for the purpose of deflection. It follows that a small distance between the movable electrodes and the substrate is here again of essential importance so as to permit the utilization of fringe effects which is required for a movement at right angles to the chip plane. Large amplitudes in the case of large lateral dimensions of the movable element can therefore not be achieved in this case either.
N. Asada describes in xe2x80x9cSilicon Micromachined Two-Dimensional Galvano Optical Scannerxe2x80x9d in IEEE Transactions on Magnetics (30) 6 (1994), pp. 4647 et seq., a drive principle making use of a magnetic moment. According to this principle, the movable element of a torsional actuator has laterally applied thereto coil-shaped conducting tracks so that a magnetic torque which can be used for driving will be generated in response to a flow of current in the magnetic field. This drive principle avoids the limitation of the amplitude caused by the geometry of the electrodes. The torque that can be realized by the magnetic moment is, however, only low so that even in the case of resonance only amplitudes of less than 3xc2x0 are achieved. In addition, the set-up is comparatively complicated and expensive due to the use of a separately manufactured permanent magnet.
It is the object of the present invention to provide a micromechanical component with an oscillating body which, making use of low voltages, permits a large deflection of the oscillating body out of the chip plane.
This object is achieved by a micromechanical component comprising a frame layer and an oscillating body which, with the aid of a suspension means, is supported in an opening penetrating the frame layer, in such a way that the oscillating body is adapted to be pivoted about an axis of oscillation vertically to the frame layer plane. At least one oscillating-body lateral surface, which extends substantially at right angles to the frame layer plane, is arranged in relation to at least one inner lateral surface of the opening in such a way that a capacitance formed therebetween is varied by an oscillation of the oscillating body in such a way that an oscillation of the oscillating body about the axis of oscillation can be generated by periodically varying a voltage applied between the frame layer and the oscillating body. In addition, a supporting substrate for holding the frame layer is provided, the supporting substrate being implemented such that, in comparison with the influence of the voltage applied between the frame layer and the oscillating body, the supporting substrate has a negligible physical influence on the generation of the oscillation of the oscillating body.
The micromechanical component according to the present invention is based on a new kind of electrostatic drive principle permitting the generation of resonant bending and torsional oscillations of movable parts of micromechanical components, the movement of the oscillating body taking place out of the chip plane and the achievable amplitude being not limited by a counterelectrode. In the case of this drive principle the inner lateral surface of the frame layer, in which the oscillating body is supported, serves as a counterelectrode; this inner lateral surface is arranged directly adjacent a lateral surface of the oscillating body serving as a movable electrode so that a kind of plate capacitor is obtained, which is defined by the opposed lateral surfaces. The electric field extends therefore parallel to the chip plane.
The coupling in of energy required for the oscillation of the oscillating body takes place via this capacitance. The coupling in of energy is effected by applying a voltage between the movable electrode and the counterelectrode, the electric voltage being increased and reduced preferably in such a way that energy averaged over an oscillating period can be coupled in. The electric voltage applied between the movable electrode and the counterelectrode is reduced or switched off preferably during the zero passage or directly adjacent the zero passage of the oscillation. This zero passage is preferably detected by an optical method, by detecting the charge/discharge current of the deflection-angle dependent capacitance or by determining the deflection-angle dependent capacitance.
In order to facilitate an oscillation build-up of the oscillating body, asymmetries produced by the manufacturing process or by arranging a further electrode above the counterelectrode may be provided so that a periodic torque causing the oscillating body to oscillate will be generated when an electric voltage having a suitable frequency is applied. The energy which can be coupled in increases as the deflection decreases. When the respective zero passage is detected and when a generator is synchronized therewith, the system is excited at its resonant frequency with the optimum phase position. It follows that, according to the present invention, electrodes restricting the oscillation amplitude due to geometrical reasons can be dispensed with completely. Furthermore, the component according to the present invention can be produced easily making use of known methods in the field of surface mechanics.
It follows that the present invention provides a micromechanical component having a simple structural design and permitting a large deflection of the oscillating body making use of a comparatively low drive voltage. These advantages are achieved according to the present invention, since one or a plurality of lateral surfaces of the oscillating body, which serve as a movable electrode and which are arranged at right angles to the plane of the supporting substrate, co-operate with one or a plurality of inner lateral surfaces of the opening in the frame layer in which the oscillating body is supported so as to permit an oscillation of the oscillating body, an underlying substrate surface, which would restrict the oscillation amplitude, being not required.